
    //==========================================
    // File:    star.js
    // Title:   几何星空连线背景
    // Author   李少杰
    //==========================================
    
    // 可调参数
    var BACKGROUND_COLOR = "rgba(255,255,255,0)";   // 背景颜色
    var POINT_NUM = 150;                        // 星星数目
    var POINT_COLOR = "rgba(0,0,0,1)";  // 点的颜色
    var LINE_LENGTH = 10000;                    // 点之间连线长度(的平方)
    
    // 创建背景画布
    var cvs = document.createElement("canvas");
    cvs.width = window.innerWidth;
    cvs.height = window.innerHeight;
    cvs.style.cssText = "\
        position:fixed;\
        top:0px;\
        left:0px;\
        z-index:-1;\
        opacity:1.0;\
        ";
    document.body.appendChild(cvs);
    
    var ctx = cvs.getContext("2d");
    
    var startTime = new Date().getTime();
    
    //随机数函数
    function randomInt(min, max) {
        return Math.floor((max - min + 1) * Math.random() + min);
    }
    
    function randomFloat(min, max) {
        return (max - min) * Math.random() + min;
    }
    
    //构造点类
    function Point() {
        this.x = randomFloat(0, cvs.width);
        this.y = randomFloat(0, cvs.height);
    
        var speed = randomFloat(0.3, 2);
        var angle = randomFloat(0, 2 * Math.PI);
    
        this.dx = Math.sin(angle) * speed;
        this.dy = Math.cos(angle) * speed;
    
        this.r = 1.2;
    
        this.color = POINT_COLOR;
    }
    
    Point.prototype.move = function () {
        this.x += this.dx;
        if (this.x < 0) {
            this.x = 0;
            this.dx = -this.dx;
        } else if (this.x > cvs.width) {
            this.x = cvs.width;
            this.dx = -this.dx;
        }
        this.y += this.dy;
        if (this.y < 0) {
            this.y = 0;
            this.dy = -this.dy;
        } else if (this.y > cvs.height) {
            this.y = cvs.height;
            this.dy = -this.dy;
        }
    }
    
    Point.prototype.draw = function () {
        ctx.fillStyle = this.color;
        ctx.beginPath();
        ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2);
        ctx.closePath();
        ctx.fill();
    }
    
    var points = [];
    
    function initPoints(num) {
        for (var i = 0; i < num; ++i) {
            points.push(new Point());
        }
    }
    
    var p0 = new Point(); //鼠标
    p0.dx = p0.dy = 0;
    var degree = 2.5;
    document.onmousemove = function (ev) {
        p0.x = ev.clientX;
        p0.y = ev.clientY;
    }
    document.onmousedown = function (ev) {
        degree = 5.0;
        p0.x = ev.clientX;
        p0.y = ev.clientY;
    }
    document.onmouseup = function (ev) {
        degree = 2.5;
        p0.x = ev.clientX;
        p0.y = ev.clientY;
    }
    window.onmouseout = function () {
        p0.x = null;
        p0.y = null;
    }
    
    function drawLine(p1, p2, deg) {
        var dx = p1.x - p2.x;
        var dy = p1.y - p2.y;
        var dis2 = dx * dx + dy * dy;
        if (dis2 < 2 * LINE_LENGTH) {
            if (dis2 > LINE_LENGTH) {
                if (p1 === p0) {
                    p2.x += dx * 0.03;
                    p2.y += dy * 0.03;
                } else return;
            }
            var t = (1.05 - dis2 / LINE_LENGTH) * 0.2 * deg;
            ctx.strokeStyle = "rgba(0,0,0," + t + ")";
            ctx.beginPath();
            ctx.lineWidth = 1.4;
            ctx.moveTo(p1.x, p1.y);
            ctx.lineTo(p2.x, p2.y);
            ctx.closePath();
            ctx.stroke();
        }
        return;
    }
    
    //绘制每一帧
    function drawFrame() {
        cvs.width = window.innerWidth;
        cvs.height = window.innerHeight;
        ctx.fillStyle = BACKGROUND_COLOR;
        ctx.fillRect(0, 0, cvs.width, cvs.height);
    
        var arr = (p0.x == null ? points : [p0].concat(points));
        for (var i = 0; i < arr.length; ++i) {
            for (var j = i + 1; j < arr.length; ++j) {
                drawLine(arr[i], arr[j], 1.0);
            }
            arr[i].draw();
            arr[i].move();
        }
    
        window.requestAnimationFrame(drawFrame);
    }
    
    initPoints(POINT_NUM);
    drawFrame();